(1) Field of the Invention
This invention relates to the formation of high quality zinc oxide films in the fabrication of electronic and optoelectronic devices, and more particularly, to the growth of zinc oxide on epitaxial lateral overgrown (ELO) gallium nitride templates.
(2) Description of the Related Art
As a direct band gap semiconductor with a room temperature energy gap of 3.37 eV, zinc oxide presents interesting electrical, optical, acoustic and chemical properties, which may find wide applications in the fields of optoelectronics, sensors and catalysis. With a large exciton binding energy (60 meV) [R. D. Vispute, V. Talyansky, S. Choopun, R. P. Sharma, T. Venkatesan, M. He, X. Tang, J. B. Halpern, M. G. Spencer, Y, X. Li, L. G. Salamanca-Riba, A. A. Iliadis and K. A. Jones, Appl. Phys. Lett. 73, 348 (1998).] and low power thresholds [D. C. Reynolds, D. C. Look, and B. Jogai, Solid State Commun. 99, 873 (1996).], zinc oxide is also being considered as a promising material for UV and blue light emitting devices. [M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, Science 292, 1897 (2001); M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Sakurai, Y. Yoshida, Z. K. Tang, P. Yu, G. K. L. Wang, and Y. Segawa, Mater. Sci. Forum 264, 1459 (1998).; D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and T. Goto, Appl. Phys. Lett. 70, 2230 (1997).] Epitaxial zinc oxide films have been grown on sapphire by several groups [M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Sakurai, Y. Yoshida, Z. K. Tang, P. Yu, G. K. L. Wang, and Y. Segawa, Mater. Sci. Forum 264, 1459 (1998).; D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and T. Goto, Appl. Phys. Lett. 70, 2230 (1997).; V. Srikant, V. Sergo, and D. R. Clarke, J. Am. Ceram. Soc. 78,1931 (1995).] despite the high mismatch between the two structures.
U.S. Pat. Nos. 5,569,548 and 5,432,397 to Koike et al discuss growing zinc oxide on a sapphire substrate. These patents teach the addition of nickel, iron, or copper to the zinc oxide to improve lattice orientation. U.S. Pat. No. 5,815,520 to Furushima also teaches growing zinc oxide on sapphire.
Similarly to gallium nitride, zinc oxide has a wurtzite-type crystalline structure. Vispute et al. have reported the epitaxial growth of zinc oxide on gallium nitride. This combination is very interesting since the lattice mismatch between these two materials is as low as 1.9%. However, because of the large dislocation density (˜109 cm−2) in the gallium nitride grown on c-sapphire, the as-grown zinc oxide films on gallium nitride are known to contain a high density of defects, which mainly include threading dislocations. Thus, it is important to obtain zinc oxide films with high crystalline quality and low dislocation density for the realization of high-efficiency zinc oxide devices. U.S. Pat. No. 5,679,476 to Uemura et al discloses epitaxially growing a non-defect layer on a substrate. U.S. Pat. No. 6,274,518 to Yuri et al epitaxially grows gallium nitride on a substrate. U.S. Pat. No. 6,673,478 to Kato et al epitaxially grows zinc oxide on a gallium nitride layer. Kato et al uses a growth substrate wherein a plurality of the (0001) surfaces are aligned in a sequence of terraces at an inclination angle of 0.1 to 0.5 degree with respect to the growing surface. The quality of Kato's ZnO is not as high as the quality of the ZnO produced by the process of the present invention.
The epitaxial lateral overgrowth (ELO) method relies on selective epitaxy and growth anisotropy, which significantly reduces the dislocation density of gallium nitride from 108-10 to 106-7 cm−2. [T. Nishinaga, T. Nakano, and S. Zhang, Jpn. J. Appl. Phys. 27 L964 (1988).; T. S. Zheleva, O.-H. Nam, M. D. Bremser, and R. F. Davis, Appl. Phys. Lett. 71, 2472 (1997).] Y. Honda et. al have proposed Facet-Controlled ELO (FACELO-through various growth parameters to control the growth facets) and also successfully reduced the dislocation density to the same level. [Y. Honda, Y. Iyechika, T. Maeda, H. Miyake and K. Hiramatsu Jpn. J. Appl. Phys. 40 L309 (2001)] Thus, it is promising to utilize the high-quality ELO gallium nitride to obtain zinc oxide films with lower defect density. In this invention, an epitaxial growth of zinc oxide films using FACELO gallium nitride template on sapphire (0001) is reported. By employing SiO2 as a mask layer, the selective growth of zinc oxide films has been realized. Electron microscopy studies show the films are single crystalline structures with low dislocation density. Photoluminescence (PL) spectroscopy demonstrates a strong ultraviolet (UV) peak from the zinc oxide. The green emission is also effectively suppressed by the high crystalline quality of zinc oxide.
Potential applications of the invention include UV detectors, light emitting diodes, laser diodes capable of emitting blue and green light and other optical electronics applications. Other applications also include transparent conductors, dielectrics and solar cells.